Field of the Invention
The present invention generally relates to systems and methods for detecting arrhythmia and, more particularly, to systems and methods for use with a cardiac monitoring device to reduce the frequency of false detections of arrhythmia.
Description of Related Art
The heart relies on an organized sequence of electrical impulses in order to beat effectively. A normal heart beat wave starts at the sinoatrial node (SA node) and progresses toward the far lower corner of the left ventricle. A wave starting in the ventricles and resulting in a rate over 100 beats per minutes (or in uncoordinated ventricular movement) is called a ventricular tachyarrhythmia. Various devices are known in the art that utilize signal processing techniques to analyze electrocardiography (ECG) signals acquired from a patient to determine when a cardiac arrhythmia such as ventricular fibrillation (VF) or ventricular tachycardia (VT) exists.
VT is a cardiac tachyarrhythmia originating from a ventricular ectopic focus, characterized by a rate typically greater than 100 beats per minute and typically by wide QRS complexes. VT may be monomorphic (identical QRS complexes) or polymorphic (varying QRS complexes). Depending on the rate and the coordination of the ventricular contraction, a heart in the VT state may or may not produce a pulse (i.e., pulsatile movement of blood through the circulatory system). If there is no pulse or a weak pulse, then the VT is considered to be unstable and a life threatening condition. An unstable VT may be treated with an electrical shock or defibrillation.
VF is a pulseless arrhythmia with chaotic electrical activity and uncoordinated ventricular contraction in which the heart immediately loses its ability to function as a pump. VF and unstable VT are the primary arrhythmias that cause sudden cardiac arrest (SCA) and lead to sudden cardiac death (SCD).
An electrical shock to the heart can correct VF and unstable VT rhythms. A simplified understanding of the process is that an electrical shock of sufficient size can force all of the cardiac cells in the heart to depolarize at the same time. Subsequently, all of the cardiac cells experience a short resting period with the hope that the sinoatrial node (SA node) will recover from this shock before any of the other cells, and that the resulting rhythm will be a pulse-producing rhythm, if not a normal sinus rhythm.
Various devices are currently available for providing an electrical shock to the heart. For example, some implantable devices, commonly referred to as pacemakers, deliver microjoule electrical shocks to a slowly beating heart in order to speed the heart rate up to an acceptable level. Other implantable devices, commonly referred to as implantable cardioverter defibrillators, deliver electrical shocks in the range of 10 to 40 joules to correct VT or VF. Also, it is well known to deliver high energy shocks (e.g., 180 to 360 joules) with a defibrillator via external paddles applied to the chest wall in order to correct VT or VF, and prevent the possible fatal outcome of these arrhythmias.
Because time delays in applying corrective electrical treatment may result in death, implantable pacemakers and defibrillators have significantly improved the ability to treat these otherwise life-threatening conditions. Being implanted within the patient, such devices continuously or substantially continuously monitor the patient's heart for treatable arrhythmias and, when such is detected, the device applies corrective electrical shocks directly to the heart. External pacemakers and defibrillators that apply corrective electrical shocks to the patient's chest wall also are used to correct such life-threatening arrhythmias but suffer from a drawback insofar as it may not be possible to use the device to apply treatment in time during an acute arrhythmic emergency to save the patient's life. Such treatment is needed within a few minutes to be effective.
Consequently, when a patient is deemed at high risk of death from such arrhythmias, electrical devices often are implanted so as to be readily available when treatment is needed. However, some patients that have temporary or uncertain permanent risk of unstable VT and VF, or are unsuitable for immediate implantation of an electrical device may be kept in a hospital where corrective electrical therapy is generally close at hand. Long-term hospitalization is frequently impractical due to its high cost, or due to the need for patients to engage in normal daily activities.
Therefore, wearable defibrillators have been developed for patients that are susceptible to ventricular tachyarrhythmias and are at temporary or uncertain permanent risk of sudden death, or are awaiting an implantable device. Such wearable defibrillators are typically configured to provide external treatment if a life-threatening arrhythmia is detected. A wearable defibrillator is available from ZOLL Lifecor Corporation of Pittsburgh, Pa. The sensitivity of the process used to detect such life threatening arrhythmia is very high and is designed to deliver treatment to every person who requires a treatment. The tradeoff to this high sensitivity is a higher level of false-positive detection of arrhythmias due to signal noise. To reduce the possibility that a false-positive reading might trigger an unnecessary treatment, wearable defibrillators use an alarm sequence to alert conscious patients of an impending treatment, who by virtue of being conscious are not experiencing a lethal arrhythmia, and allows them to stop the treatment. However, due to the sensitivity of the current detection system, the user may ignore the alarm, may not hear the alarm, may not be able to respond to the alarm, and/or may forget to depress the button to stop treatment.
In addition, other types of defibrillators, such as automated external defibrillators (AED) and implantable defibrillators, also monitor the ECG signal of a patient to determine whether a cardiac event has occurred. For instance, a typical AED includes a system for recognizing VT and VF and performing ECG analyses at specific times during a rescue event of a patient using defibrillation and cardio-pulmonary resuscitation (CPR). The first ECG analysis is usually initiated within a few seconds following attachment of the defibrillation electrodes to the patient. Subsequent ECG analyses may or may not be initiated based upon the results of the first analysis. Typically, if the first analysis detects a shockable rhythm, the rescuer is advised to deliver a defibrillation shock. Accordingly, it would be beneficial for such a system to include a signal processing routine that determines whether the detected arrhythmia is an actual arrhythmia or simply caused by noise so that a shock is only delivered to the patient if he/she is experiencing a ventricular tachyarrhythmia.
In addition, a typical implantable defibrillator detects physiological changes in patient conditions through the retrieval and analysis of signals stored in an on-board, volatile memory. Typically, these devices can store more than thirty minutes of per heartbeat data recorded on a per heartbeat, binned average basis, or on a derived basis from which can be measured or derived various measures of physiologic activity; for example, atrial or ventricular electrical activity, minute ventilation, patient activity score, and the like. From this information, a cardiac event can be detected and the system can determine whether to deliver a therapeutic shock. However, in order to conserve power in the implantable device and to ensure a therapeutic shock is delivered only when an actual cardiac event is occurring, it would be beneficial for the system to include a signal processing routine to distinguish a cardiac event from noise.
Accordingly, while the above-described defibrillators have proven very effective, a need has arisen for a detection method and system that reduces the frequency of false detections of life threatening arrhythmia by more accurately determining the difference between noise in the ECG signal and a life-threatening arrhythmia.